The present invention relates to pneumatic brake control valves for railway freight cars and particularly to an improvement thereof in terms of a release insuring feature.
Control valve devices, such as the well-known ABD type, employ a service valve assembly that is operable between brake release and application positions in accordance with the predominance of either brake pipe pressure that acts on one side of a piston member of the service valve assembly, or auxiliary reservoir pressure acting on the opposite side. The service valve assembly further includes a graduating valve that is carried by the piston member for direct movement therewith and a slide valve that is carried by the piston member with lost motion therebetween. Accordingly, relative motion exists between the graduating valve and slide valve during initial movement of the piston member in both the application and release directions, such relative motion effecting the valve connections as required to provide the various brake control functions of the service valve assembly.
A bias force in the form of a leaf spring that acts laterally on the slide valve provides face-to-face sealing pressure at the interface of the slide valve and its seat in the slide valve bushing. The slide valve is also pressed to its seat by air pressure. This combined spring and air load on the slide valve causes frictional resistance to movement of the slide valve, which the piston member encounters through its lost-motion connection with the slide valve during initial movement from release position toward application position, and again during movement of the piston member from application position toward release position. In the latter instance, the piston member encounters this resistance of the slide valve at a location intermediate release and application positions to establish a lap position of the piston member in which the supply and release of brake pressure is cut off.
This frictional resistance of the slide valve determines the differential force required to move the piston member from lap position to release position. Typically. this release differential corresponds to a 1.5 psi pressure differential by which the brake pipe pressure on one side of the piston exceeds the auxiliary reservoir pressure on the opposite side. This release differential is critical in terms of assuring a prompt release of the brakes on the cars of a train. It will be appreciated that the greater the release differential requirement to overcome the slide valve friction and effect movement of the piston member to release position, the greater the delay in effecting release of the train brakes. Factors that influence this frictional resistance include such variables as manufacturing tolerances, lap finish of the slide valve/bushing interfaces, wear of critical components, etc.